Advances in optical sciences have recently been widely recognized for their impact in the field of communications. These advances have precipitated innovation towards an all-optical network, which includes; sources, modulators, wavelength division multiplexers, amplifiers and functional optical devices. Such an all-optical network would provide increased bandwidth. However, barriers still exist that prevent the total realization of an all-optical network. One key problem for both telecommunications and data communications in an all-optical environment is in the area of integration, i.e. being able to integrate and connect a myriad of optical devices in a confined space. In this regard, the increasing sophistication of the network leads to greater complexity. More network elements—such as multiplexers, de-multiplexers, lasers, modulators, etc.—need to share the limited space available on a substrate or semiconductor chip. Thus, in order to implement a fully optical network, it becomes increasingly important to integrate multiple optical elements on a single substrate/chip in various combinations depending on the application.
The need for hardware to address high-speed data communication and information transfer has prompted the emergence of optical interconnection technology that includes multi-chip module packaging based on free space and guided wave interconnects. Integrated optical interconnection has inherited a mature and compatible technology from microelectronics, and semiconductor lasers. The optical interconnects encompass intra-chip, multichip module, inter-board, and system-to-system level links. A considerable effort has been dedicated towards the development of reliable, high speed and efficient optical links. Several areas have been developed in parallel; for example, surface-emitting semiconductor lasers, both vertical and horizontal cavity laser arrays, high-speed photodetectors, and drive circuitry. For example, 2 Gbs Vertical Cavity Semiconductor Laser (VCSEL) arrays operating at 850 nm and fast photodetectors are commercially available. These interconnections have predominately been made with short optical fibers called “jumpers,” or patch cords. However, the increasing density of these cable connections is creating problems in cable management, resulting in incorrect or unidentified connections. Other trouble spots include reliability, cost, and space consumption.
Major advances have been achieved in the technology of fabricating an optical interconnect in polymeric film in recent years. One type of such optical interconnect is an optical waveguide. The optical waveguide formed in a polymeric film is beneficial in providing short distance optical interconnections. While these waveguides have exhibited optical loss of less than about 0.1 dB/cm at a wavelength of about 850 nm, typically, high loss is associated with such optical waveguides particularly at communication wavelengths of about 1300 to about 1600 nm. For this reason their implementation has been limited to relatively short distances. See for example, “Recent Progress in Short Distance Optical Interconnects”, J. Bristow et. al., SPIE Vol. 3005, pp. 112–119, 1997.
In addition, one application in which optical interconnections would be desirable is backplane design. While it would be desirable to have a backplane design that is all-optical, the all-optical interface for backplane application does not exist because lasers, photodetectors and the like are formed of different materials. Therefore, the integration and packaging of such backplanes have been and still remain a major challenge. In spite of various successful endeavors, a fully new approach is needed to easily integrate the three main components; the waveguide, the optical source and the optical detector of an optical interconnect unit.
Sol-gel waveguide structures on silica or silicon substrates have been demonstrated previously in the form of discrete passive optical components, such as, wavelength filters or splitters. Due to the physical properties of such substrates, the size of such components is typically small, one inch or so, which is perfectly acceptable for discrete components, but is of no value in backplane applications where the optical paths of interest are tens of inches.
Traditional work in the area of sol-gel based interconnects/waveguides have required high temperature processing and, therefore has resulted in devices having high optical loss. For example, traditional sol-gel based interconnects are typically fabricated in the about 800 to about 1000 degree Celsius range. As such, the underlying substrate is limited to materials that can withstand this high temperature processing, such as glass. Additionally, rare earth ion (erbium, ytterbium, etc.) doping of sol-gel to create active regions within the waveguides has been limited to traditional high temperature processing. See for example, “Fabrication of Highly Concentrated Er3+ Doped Aluminosilicate Film Via Sol-Gel Processing”, Ryu et. al., Applied Physics Letters, Vol. 66, p. 2497, 1995. In this erbium doped sol-gel application, the post-bake anneal processing involves temperatures ranging from about 650 to about 1050 degrees Celsius. Also, see “Preparation and Characterization of Sol-Gel Derived Er3+: Al2O3—SiO2 Planar Waveguides”, Benatsou et. al., Applied Physics Letters, Vol. 71, p. 428, 1997. In a similar fashion, this erbium doped sol-gel application requires post-bake annealing at temperatures in the range of about 900 to about 1100 degrees Celsius.
Recently, the use of hybrid materials (i.e. sol-gel and polymer combinations) has resulted in sol-gel based interconnects capable of being formed using conventional microelectronic fabrication techniques, such as conventional photolithography. For an example of such hybrid materials see, “Sol-Gel Waveguide Fabrication Parameters: An Experimental Investigation”, Du et. al., Journal of Optical Engineering, Vol. 37, pp. 1101–1104, 1998. However, the resulting interconnects result in optical loss greater than about 2.0 dB/cm. This degree of optical loss can be attributed to the processing temperatures that are characteristically in excess of the glass transition temperature of the sol-gel based material. Additionally, the hybrid materials do not lend themselves to conventional doping processes; i.e., ion implantation and the like.
Conventional optical waveguides have been implemented in many applications, such as sensor applications. See for example, “Integrated Optical Sensors Based on Reactive Low Voltage Ion Plated Films”, Kunz et al., Eurosensors IV, Karlsruhe, 1990. These sensors are produced with a reactive ion plating of Ta3O5 and exhibit a refractive index of about 2.2 at a loss of about 1.1 dB/cm for TE0 mode and about 1.3 dB/cm for TM0 mode. This type of sensor resides in a substrate and is, therefore, limited in that it cannot be properly integrated in the composite optoelectronic structure that it functions along side. In another application glass waveguides formed in organic substrates have been developed for sensor applications. See for example, U.S. Pat. No. 5,480,687, entitled “Optical Waveguide with a Substantially First Substrate and a Process for its Production”, issued on 2 Jan. 1996, in the name of inventors, Heming, et al. The waveguides disclosed in the Heming '687 patent are limited in that they exhibit attenuation loss of about 5 dB/cm. The industry desire is to provide for waveguides and optical sensing units with attenuation loss approaching a totally loss-less device.
As capacity demands escalate in the high-speed data communication industries, service providers face competitive pressure to deliver data at the lowest cost per bit per kilometer, rapidly provision new services, and reconfigure bandwidth to meet customer requirements. Presently, data communication industries are experiencing explosive growth in the demand to transport large volumes of data. This demand is largely the result of the impact of Internet and multimedia communication for business, education, and recreation. Not only are the numbers of users increasing rapidly, but the per-user demand for bandwidth is increasing with computer technology advances as well. At present, the standard bandwidth for an optical fiber communication link is 2.5 Gbit/s. Some systems of 40 Gbit/s are beginning to be deployed at present, but predictions expect that systems operating at more than 1 Tbit/s will be required in the near future.
Wavelength division multiplexing (WDM), in which data is transmitted at a lower bit rate, over multiple wavelengths or channels is one solution to this problem. For example, sending signals having four different wavelengths (channels) through the same fiber each at 2.5 Gbit/s would increase the system bandwidth to 10 Gbit/s. In recent years, 8- and 40-channel and even 80-channel DWDM (Dense WDM) systems have become commercially available, thus paving the way for DWDM deployment. However in known DWDM systems, the number of channels may be limited due to the limited gain bandwidth of Erbium Doped Fiber Amplifiers (EDFAs) and the problems inherent with wavelength-selective Distributed Bragg Reflector (DBR) lasers.
Due to these bandwidth limitations current optical-to-electrical-to-optical techniques will have difficulty meeting the needs of this new network. In the future, many network functions will take place in the emerging optical layer. The functions will require all-optical-network elements such as, add/drop filters, switches that, in conjunction with DWDM, will revolutionize communications networks. All-optical network elements will eliminate bottlenecks and create more service delivery options.
Early work in the area of integrated optics focused on known theoretical concepts established previously in the microwave field. In this regard, early technological advancements centered on developing and demonstrating devices that were compatible with rudimentary integrated optical technology. The distributed feedback (DFB) laser demonstrated by Kogelnik and Shank in 1972 was a milestone along this path. A wide variety of passive optical components such as optical directional couplers, Y-branches, waveguide crossings, acousto-optical filters, Bragg gratings, transmission gratings, optical switches and modulators were also demonstrated around that time. A 4-channel WDM transmitter represented the first attempt to realize integrated optical circuits for telecommunications.
Today, the technology of integrated optics has attained a high degree of maturity. Most of the integration applications have used materials such as indium-gallium-arsinide-phosphorus (InGaAsP), gallium-aluminum-arsinide (GaAlAs), lithium niobate (LiNbO3) and glass. InGaAsP/InP systems allow for monolithic integration in the 1.3/1.5 μm region, which is widely known to represent the attenuation window for glass fibers. The waveguide structures from this materials system are formed by epitaxy and dry etching. GaAlAs/GaAs systems allow for monolithic integration in the 0.8 μm region, which is the wavelength widely used for short-range connections for telecommunications. Lithium niobate is an anisotropic material with high electro-optical and acousto-optical coefficients; however, the resulting devices have significant polarization dependence.
Integrated optics technology is already finding wide applications in telecommunications and computer technology, and one can confidently expect that in the near future concepts like waveguides and optical network will have firmly entered the household usage. The developments of this future technology are still being carried out and improvements in this area include the need to develop integrated components and devices that minimize space consumption on the chip/substrate and accomplish this task in a cost effective manufacturing environment.